Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
The movement of the heads across the disc surfaces in disc drives utilizing voice coil actuator systems is typically under the control of closed loop servo systems. In a closed loop servo system, specific data patterns used to define the location of the heads relative to the disc surface are prerecorded on the discs during the disc drive manufacturing process. The servo system reads the previously recorded servo information from the servo portion of the discs, compares the actual position of the actuator over the disc surface to a desired position and generates a position error signal (PES) reflective of the difference between the actual and desired positions. The servo system then generates a position correction signal which is used to select the polarity and amplitude of current applied to the coil of the voice coil actuator to bring the actuator to the desired position. When the actuator is at the desired position, no PES is generated, and no current is applied to the coil. Any subsequent tendency of the actuator to move from the desired position is countered by the detection of a position error, and the generation of the appropriate position correction signal to the coil.
When power to the disc drive is lost, servo control of the current flow in the coil of the voice coil actuator is also terminated. In the absence of DC current flowing in the coil, the actuator is free to move in response to such things as mechanical shock, air movement within the disc drive or mechanical bias applied to the actuator by the printed circuit cable (pcc) used to carry signals to the coil and to and from the heads mounted on the actuator. Since a power loss also means that the spindle motor will also cease to rotate the discs, the air bearing supporting the heads also begins to deteriorate and contact will be made between the heads and the discs. Because of this, it is common practice in the industry to monitor input power to the disc drive, and, at the detection of power loss, to drive the actuator to a park position and latch it there until power to the disc drive is restored.
It is also well known to use the back electromotive force (BEMF) generated by the inertia of the spinning discs and spindle motor components to generate the power to move the actuator to a park position, and the park position is typically selected to be at a location which places the heads closely adjacent the hub of the spindle motor. By parking the heads toward the inner diameter of the discs, the amount of power necessary to overcome the frictional drag of the heads on the discs at power-up is minimized.
An alternative approach to protecting the heads and discs in the event of a power loss to the disc drive is to utilize a ramping system closely adjacent the outer diameter of the discs to remove the heads from engagement with the discs. The actuator is parked with the heads supported by the ramps and latched in this position until power to the disc drive is restored. Upon re-establishment of power to the disc drive, the actuator is unlatched, and the heads are loaded back into engagement with the discs onto an established air bearing. In disc drives utilizing such ramp loading and unloading systems, the heads and discs should never come into direct contact.
The present invention may be implemented in either a disc drive which parks the heads in contact with the discs at a predefined park zone or in a disc drive which parks the heads on ramps away from direct contact with the discs.
The principal requirements of an actuator latch mechanism are that it hold the actuator at the park position in the presence of a defined maximum specified amount of mechanical shock during the time interval when power is not applied, and that the latching mechanism be capable of releasing the actuator once power has been reapplied to the disc drive and the spindle motor brought back up to operational speed. Many forms of latches to hold the actuator at the park position have been used and are disclosed in the art. These include magnetic latches, solenoid-activated latches, shape-memory metal latches and aerodynamically activated latches. For a representative review of several prior art actuator latches, the reader is directed to U.S. Pat. Nos. 5,612,842, issued Mar. 18, 1997, 5,581,424, issued Dec. 3, 1996, 5,555,146, issued Sep. 10, 1996, 5,365,389, issued Dec. 15, 1994, 5,361,182, issued Dec. 1, 1994, 5,313,354, issued May 17, 1994, 5,262,912, issued Nov. 16, 1993 and 5,231,556, issued Jul. 27, 1993, all assigned to the assignee of the present invention and all incorporated herein by reference.
Another type of actuator latching mechanism that has seen increasing usage is referred to generally as an inertial latch. Examples of inertial latching mechanisms for the actuator in disc drives are described in detail in U.S. Pat. No. 5,296,986, issued Mar. 22, 1994 to Morehouse, et al., U.S. Pat. No. 5,404,257, issued Apr. 4, 1995 to Alt, U.S. Pat. No. 5,612,842, issued Mar. 18, 1997 to Hickox, et al. and U.S. Pat. No. 5,623,384, issued Apr. 22, 1997 also to Hickox, et al. The latter two of these references are also assigned to the assignee of the present invention and incorporated herein by reference.
In a typical inertial latch, such as those described in the above cited references, the inertial element used to contact and hold the actuator against movement as a result of applied mechanical shock is a completely passive device. That is, it does not contact the moving portion of the actuator in any way until a mechanical shock is experienced by the disc drive. Because of this, it is common practice in the industry to employ a secondary latching device in the disc drive. This secondary latching device is used to hold the actuator in its designated park position in the presence of small internally generated forces which would tend to move the actuator from its park position onto the portion of the disc surface used for data recording. Examples of such small internally generated forces include, but are not limited to, 1) any bias force exerted on the moving portion of the actuator by the pcc used to electrically and electronically connect the moving portion of the actuator to circuitry on the non-moving portion of the disc drive; 2) any aerodynamic force applied to the moving portion of the actuator as a result of the spinning of the discs; and 3) any frictional force, felt between the heads and spinning discs when the discs are spinning too slowly to fly the heads in their normal manner, which tends to move the actuator moving portion to a position, usually associated with the approximate middle of the data recording area of the discs, where the frictional force is tangent to a data track and on a line that passes through the pivot axis of the actuator. A person of skill in the art will appreciate that such forces are relatively small in relationship to the amounts of applied mechanical shock that a disc drive of the current generation is expected to experience without allowing the actuator to unlatch. As an example, typical disc drives which are intended for use in laptop computer systems are specified to withstand applied mechanical shocks of 200 G or greater without damage or disengagement of the actuator from an intended park position.
Because the secondary latching mechanism of a disc drive is intended primarily to overcome only small internally generated forces, such as those described above, the burden for maintaining the moving portion of the actuator at the latch position in the presence of specified amounts of applied mechanical shock is assumed by the inertial latching element in an inertial latching system. Because, ideally, the inertial element is completely passive in the absence of applied mechanical shock, it is common that some spacing, or clearance, exist between the contact surface of the inertial element and the corresponding contact feature on the moving portion of the actuator, in order to allow for manufacturing tolerances in the associated components and to assure that the inertial element can move into its latching position relative to the moving portion of the actuator in response to experienced mechanical shock. The inclusion of such a clearance in an inertial latch design, however, allows for unintended unlatching of the actuator under certain conditions.
The following discussion assumes, as an example, that the disc drive includes a relatively weak magnetic secondary latching mechanism for holding the actuator at the park position in the presence of relatively weak internally generated forces, such as those noted above. When power to the disc drive is removed, it is common in the industry, as noted above, to use the BEMF generated by the spindle motor to provide power to move the actuator to its park position, as noted above. However, at some point in the powering down of the disc drive, there will not be sufficient BEMF to hold the actuator at its park position. The secondary latching mechanism however is engaged once the actuator is initially moved to the park position, and will be strong enough to hold the actuator at the park position against the small internally generated forces.
When a relatively large mechanical shock is applied to the disc drive in a direction which tends to move the actuator away from the latched position toward the data recording area of the discs, the inertial element of an inertial latching system moves in response to the mechanical shock into a position where it will engage the moving portion of the actuator and hold it against the mechanical shock. If, however, the clearance or spacing noted above is present in the inertial latch design, it is possible for the actuator to disengage from the secondary latching mechanism, and there is no force present in the system to move the actuator back into engagement with the secondary latching system upon termination of the applied mechanical shock. Once the mechanical shock event ends, the inertial element of the inertial latching system moves back out of engagement with the actuator to its quiescent passive position, and the actuator, freed from the secondary latching mechanism, is subject to movement onto the data recording portion of the discs by the internally generated forces. Indeed, this exact behavior has been observed with certain designs of inertial latches through the use of high speed video recording.
Similarly, applied mechanical shock exerted on the disc drive in a direction tending to force the actuator toward the latch position and away from the data recording area of the discs can also result in unintended unlatching of the actuator. This is because, as noted above, the limit stops that define the nominal extremes of the range of motion of the actuator include some amount of compliance in order to provide a "soft" stopping of the actuator and to prevent damage to the delicate gimbal assemblies used to support the heads. If the applied mechanical shock causes the compliance in the limit stop to be taken up, upon termination of the mechanical shock event, the resilience in the limit stop acts to drive the actuator back toward the data recording area of the discs, and, if the force exerted by the compliant limit stop is great enough, the secondary latching mechanism can be overcome, once again leaving the actuator free to move onto the data recording portion of the discs in response to the internally generated forces. This type of unintended unlatching of the actuator has also been observed through the use of high speed video recording.
Clearly a need exists for a latching system to hold the actuator of a disc drive at a park position, and maintain the actuator at the park position in the presence of any specified applied mechanical shock, and which is passive in the absence of mechanical shocks applied to the disc drive, and overcomes the shortcomings of prior art inertial latching systems noted above.